Diodes are semiconductor devices essential to power converters such as converters or inverters, along with switching devices such as insulated gate bipolar transistors (IGBTs) and metal-oxide semiconductor field-effect transistors (MOSFETs). The field of application of these power converters has been spread to not only industrial and household electrical equipment but also other equipment including transportation equipment such as railway vehicles and automobiles and power transmission and distribution equipment of electric power systems, and there has been demand to increase electric power and reduce loss in semiconductor devices such as diodes and switching devices.
Accordingly, in spite of the fact that the dominating semiconductor devices have conventionally been those using silicon (Si), semiconductor devices using a semiconducting material that has a larger band gap than Si, such as silicon carbide (SiC) or gallium nitride (GaN), have been developed. In recent years, semiconductor devices using gallium oxide (Ga2O3) have been developed, the gallium oxide being an oxide semiconductor that has a larger band gap than SiC and GaN and serving as a semiconducting material that is expected to further increase electric power and reduce loss.
A conventional semiconductor device using gallium oxide configures a Schottky barrier diode using gallium oxide, in which a cathode electrode is provided on one surface of an n-type gallium oxide substrate that contains n-type impurities to be ohmic-joined, an n-type gallium oxide layer with a lower n-type carrier density than the n-type gallium oxide substrate is provided on the other surface of the n-type gallium oxide substrate, and an anode electrode is provided on the n-type gallium oxide layer to be Schottky joined (see, for example, Patent Document 1).
Incidentally, as compared with PN diodes, Schottky barrier diodes can in principle reduce forward voltage and thus may be used to improve the efficiency of power converters in high power applications where a large current flows into diodes. Also, Schottky barrier diodes are unipolar devices and capable of switching faster than PN diodes, and thus they may be used to reduce the sizes of power converters by increasing switching frequencies. In particular, when SiC is used as a semiconducting material, the reverse withstand voltage for the case where a reverse bias is applied can be increased because SiC has a larger band gap than Si, and accordingly Schottky barrier diodes that have reverse withstand voltages of approximately several kilovolts have been put into practical use.
Conventional semiconductor devices using SiC include a p-type semiconductor region formed adjacent to and around a Schottky junction portion of an n-type SiC layer that is Schottky joined with an anode electrode, and this structure allows the provision of a termination structure that includes a PN junction. Schottky barrier diodes with improved reverse withstand voltages have been configured in this way.
In the termination structure of a conventional Schottky barrier diode using SiC, the p-type semiconductor region is formed by doping the n-type SiC layer doped with n-type impurities with p-type impurities and thereby making the p-type impurity concentration in the SiC layer higher than the n-type impurity concentration therein (see, for example, Patent Document 2).
Meanwhile, PN diodes using an oxide and PIN structures that include an insulation layer at the PN interface have conventionally been proposed. A structure that inserts an i-type semiconductor layer into the interface of the PN junction has the advantages of increasing the expanse of a depletion layer when a reverse voltage is applied and obtaining high-speed response characteristics as a device (see, for example, Patent Document 3).